When connected to a Lynx, why does the GPS Accuracy in ForeFlight Mobile sometimes show as 30m or 10 m?

When connected to a Lynx, the GPS accuracy value provided by the Lynx and shown within ForeFlight Mobile is based upon the Navigation Accuracy Category for Position (NACp) standard. The NACp value is determined by the GPS accuracy of the aircraft’s horizontal GPS position information.

The NACp value specifies, with 95% probability, that the reported position information is correct. A minimum NACp value of "8" must be transmitted in order to operate in airspace defined in 14 CFR 91.225. The table below explains the acceptable NACp values 8-11 and the corresponding value that is shown within ForeFlight.

NACp Value

GPS Accuracy Limits

GPS Accuracy Value Shown in ForeFlight

8

Value between 92.6m and 30m

92.6m

9

Value between 10m and 30m

30m

10

Value between 3m and 10m

10m

11

Value less than 3 meters

3m

Table 1. NACp values, accuracy limits, and what is shown within ForeFlight.

Seeing GPS accuracy values of either 92.6m, 30m, 10m, or 3m within ForeFlight is normal and within acceptable NACp limits.

Of note, in L3 test flights, GPS accuracy in ForeFlight Mobile (and on the GPS status screen of the NGT-9000) was displayed as 30m (NACp = 9) as well as 10m (NACp = 10).